With the development of broadband communication technology in wireless communication systems such as a digital microwave wireless communication system, improved of the transmission capacity (transmission throughput) of data signals is being sought. One technology that enables an improvement of transmission capacity adopts multivalued modulation of data signals to increase the number of bits per symbol of a modulated signal in which a data signal has been modulated and thus raises the efficiency of the use of frequency. Symbols are the units of modulated signals that can be transmitted at one time, and the number of bits per symbol of a modulated signal is referred to as the modulation multivalue number.
When modulation is multivalued, however, the phase and amplitude of each symbol of the modulated signal approach each other and the modulated signal becomes more prone to influence from the state of the transmission line. As a result, when the state of the transmission line is poor, the error rate of the modulated signal increases, resulting in a still further decrease of the transmission capacity of the modulated signal.
As a solution to the above-described problem, recent years have witnessed the use of an adaptive modulation method that adaptively switches the method of modulating data signals according to the state of the transmission line. In an adaptive modulation method, a modulation method in which the modulation multivalue number is small is used when the state of the transmission line is poor, and a modulation method in which the modulation multivalue number is large is used when the state of the transmission line is good. As a result, a modulation method that is appropriate to the state of the transmission line can be used, and the transmission capacity of the data signal can be improved.
In the adaptive modulation method, a wireless device in the wireless communication system transmits, in addition to the data signal, a pilot signal that indicates the modulation method of the data signal that is received from the wireless device of the communication destination. The wireless device of the communication destination transmits a data signal that is modulated by the modulation method indicated by this pilot signal. In this way, synchronization of the modulation methods can be established between wireless devices even if the modulation method of the data signal changes, whereby the data signals can be demodulated.
Nevertheless, when an error occurs in the pilot signal, synchronization of the modulation methods cannot be achieved, resulting in the potential inability to demodulate the data signals. As a result, not only is the pilot signal given an error correction code having even greater redundancy than the data signal, but the pilot signal is also modulated by a modulation method in which the modulation multivalue number is relatively small, whereby the modulation method is less prone to influence from the state of the transmission line. For example, the pilot signal is modulated by QPSK (Quadrature Phase Shift Keying), which is a modulation method having a modulation multivalue number of “2.”
In addition, a wireless device must estimate the amplitude and phase of a data signal in order to demodulate the data signal. Because a pilot signal does not include information relating to the amplitude and phase of this data signal, a pseudo-pilot signal that is separate from the pilot signal may also be transmitted to improve the accuracy of estimating the amplitude and phase of the data signal.
For example, the data transmitter described in Patent Document 1 converts a predetermined symbol in the data signal to a pseudo-pilot signal that indicates the amplitude and phase of the data signal and then transmits. The method of modulation of the pseudo-pilot signal is determined such that it has a modulation multivalue number that is no greater than the modulation multivalue number of the data signal. In addition, if the modulation multivalue number of the modulation method of the pseudo-pilot signal is no greater than the modulation multivalue number of the modulation method of the data signal, the modulation method of the pseudo-pilot signal may also be fixed and may also change together with time.